As is well known in the field of optics, high power laser beams, while useful for many purposes, are potentially dangerous to surrounding instrumentations as well as to the sensory organs of the technicians using them. Accordingly, there has been a long felt need for protective means to guarantee safety to such instruments and operators.
Nonlinear optical materials have been proposed for such protective means but to date have proven to be inadequate for most applications for a number of reasons including low magnitude nonlinearity, low damage threshold, and environmental instability.
Substantial second- and third-order nonlinear optical effects have been demonstrated for organic molecules, crystals, powders and polymers in recent years. Delocalized electrons on the organic compounds are the basis for the large nonlinear effects exhibited by these materials. These electrons are sensitive to external stimulation and can transfer their response over long distances within molecules. Other features in organic molecules also play a part in determining their nonlinear response. According to J. Zyss in Journal of Non-Crystalline Solids, 47, p. 211 (1982), these structural requirements may be outlined as follows.
Conjugation. In organic molecules, the presence of a highly delocalized -electron system considerably increases the optical nonlinearity of these molecules compared with related nonconjugated compounds, e.g., in an aromatic ring, or as developed by the resonant alternation of single and multiple chemical bonds in polyacetylenes or polydiacetylenes.
Presence of charge transfer. Substitution of an electron-attracting group, such as -NO.sub.2, and an electron-donating group, such as -N(CH.sub.3).sub.2, at two electronically interacting sites of the same molecule increases molecular nonlinearity.
Lack of centrosymmetry. The absence of a center of inversion symmetry is essential for the manifestation of even harmonics generation in crystals. Generally, chemists have found that acentricity in a crystal can best be ensured by including chirality in the individual molecules. The property of chirality enables a compound to exist as a pair of nonsuperimposable mirror images (enantiomers). A pure enantiomer will crystallize with an acentric unit cell and, accordingly, has the potential for second-harmonic generation and other .chi..sup.2 effects.
To date, purely organic polymeric compounds have received almost all of the emphasis in the search for macromolecular systems with large third-order susceptibilities. U.S. Pat. Nos. 4,431,263 and 4,536,450, issued to Garito, for example, disclose nonlinear optical materials which are based on polymers formed from diacetylenic species.